He said some of Mr Blair's comments in the run up to war "stretched the truth".

Mr Meacher's campaign claims to have the backing of "about 30" Labour MPs, although he said he was not prepared to name them at this stage.

Leadership candidates need 45 signatures from Labour MPs to get on to the ballot.

But Labour MP Stephen Pound, who described Mr Meacher as a "faintly ridiculous" figure, said he would be surprised if the veteran left-winger managed to get the required support.

The chancellor is the favourite to succeed Mr Blair

"It's a pity because, frankly, he's going to get humiliated," said the Ealing North MP.

"I suspect he knows that, if he put his ego to one side a bit he would realise that."

BBC political editor Nick Robinson said some Labour MPs would welcome a contest and Gordon Brown himself might prefer a contest to a "coronation", as it would give his leadership more of a mandate.

But asked whether a contest would be in the best interests of the Labour Party on Thursday, Mr Brown replied: "That's up to other people".

He welcomed any debate about the future of the party and said: "The key thing is making our country great in the future by making the right decisions now."

I have asked Michael to come on board with our campaign, but he's chosen to go his own way

Left-wing leadership challenger John McDonnell

Mr McDonnell, who will be competing with Mr Meacher for left-wing Labour backing, said he had the support of about 22 MPs, and would not drop out of the race.

He said Labour members would judge the candidates on their records, and while he had voted against tuition fees, the Iraq war, privatisation and benefit cuts, Mr Brown and Mr Meacher had consistently voted for them.

"I have asked Michael to come on board with our campaign, but he's chosen to go his own way. Now let's have the debate on policies and the election and let the members decide," he said.

Asked whether his challenge might split the left and leave neither able to take on the chancellor, Mr Meacher said he did not think Mr McDonnell could get the necessary number of nominations.

"I firmly believe that I can, and to that extent, I am putting the left into the ring and I think and hope I am uniting the left rather than splitting it."